Fiber optic moisture sensor

ABSTRACT

This invention provides for a fiber optic moisture sensor. The sensor is comprised of a housing and a support positioned within the housing. The support is coated with a film. A first and second light guides are positioned within the housing and communicate illumination to and from the film. A reflective surface is positioned within the housing facing the film. The film comprises an optically transparent polymer and a salt complex of a metal ion and an organic compound. The salt complex is capable of absorbing moisture and emits a fluorescence signal when excited by light at the appropriate wavelength. The fluorescence signal can be quenched when the salt complex absorbs moisture. An apparatus incorporating the sensor and a method of making the sensor are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This a divisional of application Ser. No. 07/914,795, entitled "FIBEROPTIC MOISTURE SENSOR" filed Jul. 16, 1992, now U.S. Pat. No. 5,319,975.

BACKGROUND OF THE INVENTION

Humidity measurements are important in many industrial operationsincluding food processing, the manufacture of plastic, paper and finechemicals and in the production of semiconductors. In severalbiochemical processes where water activity or moisture levels are vitalto the metabolic productivity of microorganisms, moisture levels must bedetermined in a fast, accurate, reliable manner, and on a real-timebasis. Over the past few years, different types of humidity sensors havebeen developed and the recent trend has been to place such sensors in anin-line system for the detection and control of moisture levels.

Generally, moisture sensors incorporate a sensing material with anappropriate transducer. Sensing of moisture is achieved by interactionof the water molecules with the sensing material and the resultingelectrical or optical signal is relayed to a detection system via atransducer. For example, Hijikigawa et. al in "A Thin-Film ResistanceHumidity Sensor", Sensors and Actuators, 1983, 4, pages 307-315 havedeveloped a humidity sensor consisting of a thin film coating on aluminasubstrate. The film is a composite made from crosslinkedpolystyrene-sulfonate covered with a protective film of cellulose ester.The film electrical resistance is sensitive to humidity.

In another example, Y. Sakai et al. "Humidity-Sensitive andWater-Resistive Polymeric Materials", Sensors and Actuators, 1988, 16,pages 359-367 discuss a two-polymer system comprisingpolytetrafluoroethylene vinylpyridine graft copolymer (PTFE-VP) andcrosslinked polyvinylpyridine with 1,4-dibromobutane. The impedance ofthe two-polymers changes with the relative humidity.

As an example of ceramic or metal oxide based sensors, Y. Sadoka et. al"Electrical Properties of Anodized Aluminium in a Humid Atmosphere",Journal of Materials Science, 1986, 21, pages 1269-1274 haveinvestigated anodized aluminium for the measurement of humidity. On theother hand, S. Mukode et al. in "A Semiconductive Humidity Sensor",Sensors and Actuators, 1989, 16, pages 1-11, discuss the use ofsemiconducting materials such as stannic and titanic oxides.

Cobalt chloride (CoCl₂) has been extensively used as a specific opticalindicator for humidity. Anhydrous cobalt chloride is blue and changes topink when hydrated. Generally, cobalt chloride based sensors havelimitations and different concentrations of cobalt chloride are neededto measure a wide range of humidity levels. Other limitations includepoor stability and reproducibility, variable accuracy, a limitedtemperature range, and a slow response time. Cobalt chloride basedsensors may not be appropriate in many industrial situations.

Russell et. al in "Optical Sensor for the Determination of Moisture",Analytica Chimica Acta, 1985, 170, pages 209-216, report embeddingcobalt chloride into a gelatin matrix and coating the matrix onto a 600μm optical fiber to produce a humidity sensing system. The change inrelative humidity is related to the absorption of light by CoCl₂ at 680nm. The light is transmitted to the fiber and is then reflectedinternally.

Ballantine et. al in "Optical Waveguide Humidity Detector", AnalyticalChemistry, 1986, 58(13), pages 2883-2885, also use cobalt chloride, inconjunction with a glass capillary tube. The cobalt chloride isincorporated into polyvinylpyrrolidone (PVP). The sensor is coupled tolight emitting diodes and calibrated by monitoring the change in lightabsorption by cobalt chloride at a given wavelength.

Zhou et. al in "Porous Fiber-Optic Sensor for High-Sensitivity HumidityMeasurements", Analytical Chemistry, 1988, 60(20) pages 2317-2320discuss a humidity sensor using cobalt chloride incorporated into aporous glass fiber with a high surface area. The sensitivity of thesensor is dependent on the concentration of cobalt chloride. At lowconcentrations of cobalt chloride, the sensor is able to measure at lowrelative humidity. By increasing the concentration of cobalt chloride,the sensor is able to measure high relative humidity.

In other applications, Harris et. al in "Colorimetric Detection ofHumidity and Other substances with Solvatochromic Dyes Dispersed inPorous Polymer Films", NASA Tech Briefs, MFS-26128, discuss thin polymerfilms containing solvatochromic dyes sensitive in their UV-Visibleabsorption spectrum to atmospheric water content. Detection of a colorchange can be achieved visually by comparison with a standard chart orspectrophotmetically by measurment of the wavelength of absorbance.

In other applications, sensors based on fluorescence have been used.Generally fluorescence-based moisture fiber optic sensors havedemonstrated poor sensitivity especially at low relative humidity andrespond slowly to changes in relative humidity. These limitations aredue to the dynamic range of the reagents or dyes used. For example,Posch et. al in "Fiber-Optic Humidity Sensor Based on FluorescenceQuenching", Sensors and Actuators, 1988, 15, pages 77-83, report a fiberoptic humidity sensor based on fluorescence quenching. Two differentfluorescent dyes are used as humidity indicators: perylenedibutyrate andperylenetricarboxylic acid bis-imidates (PTCABs). A silica gel sheet isused to absorb the dyes and is subsequently cast onto a glass slidewhich is attached to a bifurcated fiber optic light guide to form asensing system. Sensor sensitivity is poor at the high range of relativehumidity and the response time is slow. Furthermore, gases such asoxygen and ammonia interfere with the desired, water mediated quenchingmechanism.

Zhu et al. in "A New Fluorescence Sensor for Quantification ofAtmospheric Humidity", Journal of Electro Chemical Society, 1989, 136,pages 657-570, report a fiber optic humidity sensor. The fluorescent dyerhodamine 6G (R6G) is impregnated in a Nafion polymer and is sensitiveto changes in relative humidity. The sensor has limited sensitivity atrelative humidities below 40%.

SUMMARY OF TEE INVENTION

It is an object of this invention to provide a fiber optic moisturesensor that can be applied to selectively detect moisture present in awide range of phases.

It is another object of this invention to provide an apparatusincorporating the sensor of the invention.

It is yet another object of this invention to provide a method formaking the sensor of the invention.

It is yet another object of this invention to provide a sensor with afast response time.

It is yet a further object of the present invention to provide a sensorthat is relatively stable.

These and other objects are accomplished by a fiber optic sensorcomprising a housing and a support having a first face and a second facewithin said housing. The first face of the support is coated with afilm. A first light guide and second light guides capable ofcommunicating illumination to and from said film are incorporated withinthe housing. The sensor further comprises a reflective surfaceincorporated within the housing. The film comprises an opticallytransparent polymer and a salt complex of a metal ion and an organiccompound. The salt complex is capable of absorbing moisture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a sensor of the present invention.

FIG. 2 is a cross-sectional view of the housing of a sensor of thepresent invention.

FIG. 3 is a cross-sectional view of the housing of a sensor of thepresent invention taken along the line 33--33 in FIG. 2.

FIG. 4 is a cross-sectional view of the probe housing of a sensor of thepresent invention taken along the line 44--44 in FIG. 2.

FIG. 5 is a cross-sectional view of the housing of a sensor of thepresent invention taken along the line 55--55 in FIG. 2.

FIG. 6 is a diagram of an apparatus comprising a sensor of the presentinvention, means for illumination and means for measuring illumination.

FIG. 7 is a plot of the response of a sensor of the present invention asmeasured by fluorescence in arbitrary units (a.u) as a function of time(sec) for different relative humidities (% RH).

FIG. 8 is a plot of the response of a sensor of the present invention asmeasured by inverse fluorescence in arbitrary units (a.u.) as a functionof increasing relative humidity and decreasing relative humidity .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The sensor of the present invention operates on the principle that watermolecules in the form of moisture or humidity quench the emitted signalproduced by light excitation of a salt complex of a metal ion and anorganic compound. More particulary, as it pertains to this invention,the emitted signal can be caused by fluorescence. Generally,fluorescence quenching refers to any process that results in thedecrease of fluorescence intensity or in the decrease of the quantumyield of fluorophores upon interaction with other chemical species. Moreparticularly, as it pertains to this invention, fluorescence quenchingis the result of the interaction of specific fluorophores with watermolecules. With the sensor of the present invention, detection ofmoisture as a change in relative humidity using fluorescence quenchingcan be achieved and relative humidity levels as a function offluorescence intensity can be qualified according to mathematicalrelationships known in the art, such as for example, the Stern-Volmerrelationship which linearly relates changes in inverse fluorescenceintensity and relative humidity levels.

The sensor of the present invention can be comprised of a filmcomprising a salt complex of a metal ion and an organic compound and thefilm can be coated onto a support. The salt complex of a metal ion andorganic compound can emit a fluorescence signal when it is excited bylight at the appropriate wavelength. The fluorescence signal can bequenched when the salt complex absorbs water molecules. Changes in thefluorescence signal can be measured and monitored by means forfluorescence detection which are known in the art. The water moleculescan desorb and the sensor can operate in a reversible manner.

Generally, a salt complex of a metal ion and organic compound can beselected for its ability to absorb humidity and additionally for itsability to excite and emit light at an appropriate wavelength. Morespecifically, salt complexes of a metal ion and organic compound whichcan form a hydration complex as well as contain a metal ion for which anoptical assay can be performed can be used in the sensor of the presentinvention. The optical assay can rely on fluorescence or absorbance.Metal ion salts such as aluminium, calcium and iron sulfates oraluminium, calcium, iron and cobalt chlorides can be used to provide themetal ion. Generally, optical methods for detection of the metal ion canrely on organic O,O-donating, O,N-donating and N,N-donating chelatingreagents such as for example pyrocatechol violet, morin, lumogallion,poly(macrocyclic) compounds, 1,2,4-trihydroxyanthraquinone, calcein andvarious porphyrins.

In one embodiment of the invention, an organic complex such as morin(3,5,7,2',4'-pentahydroxyflavone) can be used. Morin can form a highlyfluorescent complex with a metal ion such as for example aluminium,beryllium, a salt complex of zinc, gallium, indium and scandium. Morespecifically, the aluminium and morin salt complex can be used and ishighly fluorescent at about 505 nm when excited at about 420 nm. Whenthe salt complex of aluminium and morin absorbs water, the fluorescencecan be quenched and the change in relative humidity can be correlated tothe intensity of the quenched signal.

In another embodiment of the invention, an organic complex such aslumogallion(4-chloro-6-(2,4-dihydroxy-naphtalazo)-1-hydroxybenzene-2-sulfonic acidmonosodium salt) can be used. Lumogallion forms a highly fluorescentcomplex with a metal ion such as for example aluminium. Morespecifically a salt complex of aluminium and lumogallion can fluoresceat about 600 nm when exited at about 490 nm.

The salt complex of aluminium and morin can be formed by dissolvingaluminium sulfate and morin with a suitable solvent and then mixing byusing mechanical agitation. A solvent system can be used which comprisesa solvent such as acetone or acetone and water in a volume ratio of fromabout 1:1 to about 3:1, preferably of from about 1:2 to about 3:2 can beused. Other solvents which can be used include methanol, ethanol andisopropanol alone or in combination with water.

Following dissolution and mixing, the salt complex can be embedded intoa film, preferably comprising a polymer such as for examplepoly(vinylpyrrolidone) (PVP). The polymer can be selected such that itis optically transparent in the wavelength range. Other opticallytransparent polymers which can be used include cellulose, siliconerubber, poly(vinyl-chloride), and poly(vinylalcohol). Alternatively,polymers which are not optically transparent can also be used providedsuch polymers are permeable to water. In the case of non-opticallytransparent polymers such as for example porous Teflon and Nafion, theoptical signal can be based on surface fluorescence.

The polymer can be added to the solution mixture of the salt complexsuch that the amount of the salt complex can vary of from about 5 toabout 20% on a weight per volume basis of the solution mixture. Theratio of polymer to the salt complex can be dependent on the desiredsensitivity of the sensor and the mechanical strength of the film. Theresulting polymer solution which comprises the salt complex can form afilm upon coating onto a support. Preferably, an optically transparentsupport such as for example silica can be used. The support can be ofany shape including, for example, square or circular and its surfacearea can be only limited by the level of sensor miniaturization which isrequired.

Prior to coating, the support can be washed with a dilute solution of amineral acid, (for example hydrofluoric acid) of from about 1 to about2% on a volume basis and for about 2 minutes. After the acid wash, thesupport can be washed with water for about 5 minutes. The support can bedried and partially covered with the polymer solution using techniquesknown in the art such that a film can be deposited. The film can be ofany shape and of any thickness. The only limitation on the shape is thelevel of miniaturization required, however, the shape of the film ispreferably circular. In a preferred embodiment, the film thickness canbe of from about 10 to about 40 μm, preferably less than about 20 μm foroptimum sensor response.

In addition to the salt complex of aluminium and morin, other complexesof a metal ion and organic compound can be used. For example, a saltcomplex of berrylium and morin, aluminium and quercetin, salt complexesof a metal ion and 1-(2Hydroxy-4-sulfo-1-naphtylazo)-2-naphtol-3,6-disulfonic acid(hydroxynaphthol blue) and salt complexes of a metal ion and8-hydroxyquinoline-5-sulfonate can also be used. Other salt complexesthat can be used include complexes of a metal ion such as calcium,nickel, copper and cobalt and complexes of a metal ion and an organiccompound comprising the o,o'-dihydroxyazo functional group.

FIG. 1 shows a sensor of the present invention. The sensor is comprisedof a probe 2 incorporated into a housing 4. Probe 2 is comprised of asupport 6 covered with film 8 and first and second optical fibers 10 and12. Support 6 is positioned flush with the first end of first opticalfiber 10 which is capable of communicating excitation light to film 8and is positioned flush with the first end of second optical fiber 12which is capable of communicating emission light from film 8.

Optical fibers 10 and 12 can be used either singly or in multitude as abifurcated optical fiber bundle. Generally, optical fibers 10 and 12 canbe selected such that they transmit light within the appropriatewavelength range based on the salt complex used.

For example, when a salt complex of aluminium and morin is used a fiberoptic bundle (manufactured by EOTec Corporation, New Haven, Conn.) witha core diameter of 48 μm, a clad diameter of 50 μm, a buffer diameter of1.78 mm and a numerical aperture of 0.56 can be used.

To enhance light communication into first and second optical fibers 10and 12 as shown in FIG. 1, optionally, a reflective surface such as forexample mirror 14 can be used. Mirror 14 can be positioned facing andparallel with film 8. Generally, mirror 14 can be selected such that thedistance 16 between film 8 and mirror 14 is about equal to the focallength of mirror 14. Mirror 14 can be of any size and focal length,however both the size and focal length of mirror 14 are limited by thelevel of miniaturization required. In a preferred embodiment, whenmirror 14 is used, it can reflect back the light which is communicatedfrom film 8 into the first end of second optical fiber 12.

FIG. 2 is a cross-sectional view of housing 4 of the sensor shown inFIG. 1. Housing 4 can be manufactured using for example polypropylene,nylon or teflon or of a metal such as for example aluminium or otherreflective metal such as, for example, stainless steel. Generally, anymetal can be used provided it is inert to the particular phase whereinmoisture is being detected. Housing 4 can be cylindrical in shape and iscomprised of a first, second and third sections 33, 44 and 55. FIGS. 3,4, and 5 illustrate the cross-sectional views of first, second and thirdsections 33, 44 and 55 respectively. As shown in FIG. 2, first, secondand third sections 33, 44, and 55 can be connected by means of first andsecond screws 21 and 22.

In an embodiment of the invention, first and second optical fibers 10and 12 can be positioned into first opening 3 and can be stabilized bymeans of set screw 23. First opening 3 can be circular and can have adiameter at least as wide as the combined outer diameters of first andsecond optical fibers 10 and 12. First opening 3 connects with secondopening 5. As shown in FIG. 1, second opening 5 can accommodate support6 and can be sized accordingly.

Second section 44 is shown in FIGS. 2 and 4 and comprises third opening7. As shown in FIG. 2, second section 44 also comprises first and secondpassages 11 and 13 which can allow the particular phase comprising themoisture to contact film 8. Third section 55 is shown in FIGS. 2 and 5,and comprises fourth opening 9 which can optionally accommodate mirror14 as shown in FIG. 1. When mirror 14 is used, the distance 16 betweenthe outer edge of second opening 5 and the outer edge of fourth opening9 can be equivalent to the focal length of mirror 14.

Generally, with regard to instrumentation, FIG. 6 shows sensor 222 ofthe present invention, means for illuminating 61 the distal end of firstoptical fiber 10 and means 69 for measuring the illuminationcommunicated at the distal end of second optical fiber 12. Sensor 222comprises housing 4 and probe 2 as shown in FIG. 1. Means forilluminating 61 can be an illumination source such as a light source,(for example, a 100 W Xenophot HLX lamp by OSRAM, Germany). Means forilluminating 61 can emit light beam 84 which can be modulated by lightchopper 62. Light chopper 62 can be a mechanical chopper (EG&G, model196). After modulation, light beam 84 can be further modified by filter63. Filter 63 can be, for example, a narrow bandpass filtercharacterized by a maximum transmission and bandwidth and can beappropriately selected such that its maximum transmission is compatiblewith the wavelength spectrum of the salt complex.

Means for measuring 69 can be comprised of a monochromator 64 (forexample, Allied Analytical Systems, Model MonoSpec 27). Monochromator 64can screen light beam 93 which can be emitted through second opticalfiber 12 and can be used to select the desired wavelength range fromlight beam 93 for subsequent detection. Monochromator 64 can becalibrated with a light source with a known wavelength for example aHe--Ne laser and can be set at a specific wavelength. Monochromator 64can pass light beam 93 into detector 65. Detector 65 can be for examplea photomultiplier tube and can be directly connected to monochromator64. An output light beam 95 from detector 65 can be produced and can bedetected by amplifier 67. Amplifier 67 can be a lock-in amplifier (EG&Gamplifier Model 5209). Light beam 97 from amplifier 67 can be convertedto a voltage signal and passed to recording system 68.

Recording system 68 can be for example a chart recorder or a personalcomputer. When recording system 68 is a computer (for example, IBMPC/AT), it can be equipped with a data acquisition software, a generalpurpose instrument bus and a Graphics card which can be directlyconnected to amplifier 67 through a plug-in cable. Recording system 68can read the signal from amplifier 67 and can analyze it in the mannerdesired.

In a preferred embodiment, as shown in FIG. 6, sensor 222 can becalibrated to detect different moisture levels. Several calibrationtechniques are known in the art and can be used to calibrate sensor 222.For example, sensor 222 can be connected to gas flow system 79. Gas flowsystem 79 can provide gas such as air or nitrogen which can comprisedifferent moisture levels. Gas flow system 79 can be comprised of one ormore flow meters 81 and 82. For example, in one particular embodiment,flow meters 81 and 82 can operate at a flow rate of about 500 ml/min,preferably about 100 ml/min. Other flow rates can be used depending onsensor 222 and the range of calibration required. In one operation ofgas flow system 79, gas which can be either dry air or nitrogen can beintroduced at inlet 70 and can enter water reservoir 80 through line 71and can enter flow meter 81 through line 72. Gas which can becommunicated through line 71 entrains water present in water reservoir80 in the form of humidity and the gas which is now comprising adetermined level of moisture leaves water reservoir 80 through line 73and enters flow meter 82. The gas leaves flow meters 81 and 82respectively through lines 74 and 75 and combines into line 76 to entersensor 222 through opening 11. The gas leaves sensor 222 through opening13 and enters means for measuring humidity 83, for example a hydrometer.When gas flows through sensor 222, changes in the moisture level canproduce a corresponding change in the light signal as described above.Such change in the light signal can be recorded and the response ofsensor 222 to different moisture levels as measured by relative humidity(% RH) can be obtained as shown in FIG. 7. In a preferred embodiment,when a salt complex of aluminium ion and morin is used in sensor 222,the light signal can be based on fluorescence. The fluorescence isquenched by moisture. Such quenching in turn produces changes in thesignal which can be plotted using a linear relationship between inversefluorescence and relative humidity. One example of a linear relationshipis the Stern-Volmer equation as shown in FIG. 8. The linear relationshipshown in FIG. 8 can be used as a calibration curve to measure moisture.

After calibration, sensor 222 can be used also as shown in FIG. 6without gas flow system 79. A particular phase of interest whichcomprises moisture such as for example gas can be introduced into sensor222 through passage 11 and can leave sensor 222 through passage 13.Similarly, when optional mirror 14 is not used, the particular phase ofinterest can contact film 8 directly.

As described above, when the moisture present in the phase of interestcontacts the film, it quenches the fluorescence signal thereby producinga corresponding change in the light signal. Such changes in the lightsignal can be recorded and the response of sensor 222 to differentmoisture levels can be obtained as shown in FIG. 7. Results can beplotted as shown in FIG. 8 and the moisture level determined bycomparison with a calibration curve.

This invention and many of its attendant advantages will be understoodfrom the foregoing description, and it will be apparent that variousmodifications and changes can be made without departing from the spiritand scope of the invention or sacrificing all of its materialadvantages, the processes hereinbefore described being merely preferredembodiments. For example, the sensor can be useful for the measurementof humidity in many industrial processes such as prevalent in the foodindustry. The sensor can also be useful in the measurement of humidityin a steam environment. Other applications of the sensor include themeasurement of water activity in liquid phases, in which case, ahydrophobic and water vapor permeable membrane can be used to separatethe film from the aqueous solution such that the sensing material in thefilm is not dissolved in the aqueous solution. For applications of thesensor to the measurement of moisture in the gas phase, a membrane madefor example from polytetrafluoride ethylene (PTFE) can be used thusallowing the passage of gas such as air, water vapor and other volatilecompounds. In other applications, the sensor of the invention can beused to measure water activity in submerged or solid substratefermentations.

What is claimed is:
 1. A method for the measurement of the level ofmoisture in a gaseous phase utilizing an optical sensor, said sensorcomprising a film of a polymer, said polymer having embedded therein apredetermined amount of a salt complex of a metal ion and an organiccompound, wherein said salt complex emits fluorescence within adetermined wavelength range, said polymer optically transparent in saidwavelength range, and wherein said salt complex absorbs the moisture insaid gaseous phase, the fluorescence emission being sensitive toquenching by said moisture, said method comprising the stepsof:contacting said optically transparent polymer with said gaseousphase; introducing excitation light into the distal end of a firstoptical fiber and communicating said excitation light to said polymerthrough the proximal end of said first optical fiber to induce saidfluorescence emission; collecting said fluorescence emission into theproximal end of a second optical fiber; detecting said fluorescenceemission through the distal end of said second optical fiber; measuringthe quenching of said fluorescence emission; and determining the levelof moisture in said gaseous phase.